+all interpreters and all threads in a process. Any variables not documented
+here may be changed or removed without notice, so don't use them!
+If you feel you really do need to use an unlisted variable, first send email to
+L<perl5-porters@perl.org|mailto:perl5-porters@perl.org>. It may be that
+someone there will point out a way to accomplish what you need without using an
+internal variable. But if not, you should get a go-ahead to document and then
+use the variable.